Question

The least stable conformation of cyclohexane is the:
(A) Twisted chair
(B) Chair
(C) Half-chair
(D) Twist boat

Answer 1

Hint : The order of stability of any compound depends upon its interaction of its $1 - 3$ , $1 - 4$ , $H - H$ bonding Higher the number of interactions lower will be the stability. The most stable conformation has highest strength and the least stable has the most torsional strain.

Complete Step By Step Answer:
Cyclohexane has the ability to flip which permits them to have various conformers. The chair conformation is spread and staggered at all the bonds therefore there is no torsional strain. All the $C - C$ bond is very close and is free of any angle strain making it most stable conformation. The half chair form is less stable than the chair used to the presence of some strain. It is $10{\text{ }}kcal/mol$ less stable than the chair conformation.
Boat conformation has steric hindrance on their carbon 1 and Carbon 4 between two equatorial hydrogen which gives them the high torsional stress and their $C - C$ bond are also ellipse making them less stable than the chair conformers.
In both the chair and boat conformation there is presence of no angle strain giving them high stability as compared to their twisted conformation.
A process of flipping rings takes place in the cyclohexane giving another conformation as a twist conformation.
Thus, Stability order of cyclohexane conformers is as follows :
Chair conformer > twisted chair conformer > boat conformer > twist boat conformer
Therefore, the correct option is option (D) twisted chair conformation.

Note :
Twisting allows some steric and torsional strength to be relieved making the twist conformation between the $H - H$ bond at the equatorial position which reduces the energy requirement to attain a slightly stable conformation.